Medical imaging is the technique used to create images of the human body or parts thereof for clinical purposes (medical procedures that seek to reveal, to diagnose or to examine disease) or medical science (including the study of normal anatomy and physiology). Computer tomography (CT) and magnetic resonance imaging (MRI) are two of the most common approaches. These techniques generate a set of individual 2D images that can be displayed in a 3D visualization as a “volume dataset.”
Volume rendering in particular is a process by which data composed of individual slices or images that are captured from an X-ray CT scanner or MRI scanner are used to generate an image from a unique position and direction in 3D space. Traditionally, this process involves rays being fired from unique positions and directions in the dataset and examining each pixel along the trajectory. Using a previously-determined accumulation function, each pixel is blended into a final grayscale or color value, and collected to form an image.
In the fields of Radiology and Interventional Cardiology, a particularly important aspect of diagnosing and treating the patient is being able to accurately image the arteries or vessels of the body. Typically, the patient will be given an X-ray CT scan, which creates a collection of consecutive image slices that form a volume of patient data when stacked, or rendered, electronically. Several known rendering methods that allow CT data to be viewed have emerged over the years. One of the most straight-forward methods, Multi-Planar Reconstruction (MPR), creates a view aligned plane that cuts through the volume data, often at an oblique angle. Using the MPR view, the clinician can adjust the position and orientation of this plane, so as to be able to see different views of the scanned anatomy as a 2D, grayscale image.
In the field of Interventional Cardiology, the MPR plane is used to create a cross-sectional view of the artery. This cross-sectional, or lumenal, view allows the clinician to assess the presence and severity of characteristics common to cardiovascular disease, such as stenosis (lumenal narrowing) and plaque accumulation in the arterial wall.
The challenge to interpreting a grayscale cross-sectional view of the artery, is reliably determining what constitutes the open area of the artery, or lumen, versus the arterial wall. This visual ambiguity leads to widely different interpretations of the same MPR image. The currently accepted margin of error for this type of reading is 20-40%, due to the subjective nature of the interpretation. Furthermore, increasing importance is being placed on the ability to accurately assess plaque volume and composition. This has proven inherently difficult as these materials differ only subtly, or not at all, when viewed in the grayscale cross-section.
Current MPR rendering methods employ a strategy based on voxel value for data reconstruction. The grayscale values of the voxels map to specific Hounsfield Units (HUs), which are a measure of radio-density. In an attempt to overcome the shortcomings of grayscale MPR, colorized versions have been developed where Hounsfield Unit ranges that are consistent with specific tissues, structures, or materials are assigned colors. For example, 100-200 HUs, a range that is consistent with the radio-density of arterial wall tissue, could be assigned the color green, plaque might be characterized as 200-300 HUs and colored blue, while contrast fluid within the lumen might be characterized as 500-1000 HUs and colored red. Although the goal of colorizing the MPR view was to not only alleviate the subjectivity of reading the grayscale images, but also to be able to more specifically characterize the composition of the arterial cross section, it has proven unreliable. Studies have shown that HU ranges can have significant inter- and intra-scan variability based on many factors, including, but not limited to: patient body mass, scanning protocol variations, the amount of contrast used, timing of the contrast, strength of radiation of a particular scan, type of plaque present in the artery, etc. These factors make the task of assigning an accurate universal color map that is based on Hounsfield Units an ill-posed problem. As a result, the traditional grayscale MPR view representing original voxel data remains the preferred viewing method.